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Recovering from Heat Stress

As summer fades into fall and ambient temperatures drop, the negative consequences of heat stress on the dairy herd often linger. Lameness, poor reproduction, low milk production efficiency, and elevated incidence of health issues may continue on a dairy even after summer’s heat is past. Understanding what nutritional and management factors influence the severity of heat stress carryover effects, and how to mitigate these factors is crucial for the herd to move successfully into the fall feeding cycle.

An over-riding point is that high-producing cows experience heat stress whenever THI reaches 68, so be sure to keep cooling cows into late summer and fall – don’t stop heat stress abatement too soon! Even mild heat stress can be damaging to cow health in both the long- and short-term.

Summer Heat Stress and Fall Lameness

Wisconsin researchers have found that the primary behavioral responses to heat stress – reduced lying and excessive standing time – predispose the cow to greater incidence of claw horn lesions and lameness in late summer and fall (Cook et al., 2007). Factor in any problems with sub-acute rumen acidosis that may have occurred during summer’s heat, together with prolonged standing to enhance cow cooling, and we have ideal conditions for onset of fall lameness as a result of hoof and claw injury sustained during the heat stress period. Bear in mind the magnitude of the change in lying time measured in the Wisconsin research: as THI increased from 56 to only 74, lying time fell by 3 h/d and was highly correlated with claw horn lesion development.

Of course, the best way to mitigate the lingering negative consequences of heat stress on herd lameness is to cool cows effectively so that extra standing time is minimized and to feed rations – and use feedbunk management – that minimize rumen acidosis. Rations formulated to optimize digestible fiber from forage and non-forage sources of fiber will provide sufficient metabolizable energy and minimize the potential for sub-acute rumen acidosis that will only exacerbate any rumen damage that occurred during earlier bouts of heat stress.

As for cow comfort, we need to double-down on management practices that boost resting time and natural feeding patterns: ample bunk space and feed available 24/7, no overcrowding of freestalls, deep-bedded stalls, and minimal time outside the pen.

Heat Stress Leads to Lower Efficiency

The interaction between heat stress and reduced feed efficiency is well known. Heat stress encourages slug feeding and lower rumination (Tapki and Sahin, 2006). When combined with a typical lactation diet rich in rapidly fermentable carbohydrates, the rumen acid load increases excessively, rumination and buffering decline, and the rumen digesta mat becomes less well-formed. Consequently, rumen pH is low and feed retention time decreases. Overall, rumen fiber and feed digestion decrease and translate into substantially lower feed efficiency. Related to the negative effects of heat stress on digestive efficiency and feed intake, it is common to observe a “milk yield slump” following heat stress that lasts through September and October.

The severity of the heat stress and how fermentable the diet was will dictate how much residual damage to the rumen may exist in late summer. It is well-characterized that recurrent episodes of rumen acidosis result in increasing severity of the pH response and associated feeding and sorting behaviors (Dohme et al., 2008). More needs to be learned about how much time is required for cows to recover from recurring bouts of sub-acute rumen acidosis, but feeding a diet designed to enhance rumen fiber digestion and the rumen digesta mat ought to mitigate lingering effects of heat stress on rumen and digestive efficiency. Focus on meeting the needs for digested and undigested NDF, physically effective NDF, and rumen fermentable carbohydrates (but with a focus on fermentable fiber; see Table 1 for recommended ranges).

Heat Stress and Fall Reproductive Performance

Heat stress negatively impacts production and reproduction partly by reduced feed intake, but also by altering endocrine and immune status, reducing rumination and nutrient absorption, and increasing maintenance requirements (Baumgard et al. 2006). Overall, less energy is available to the cow, she falls into negative energy balance, and consequently may lose substantial amounts of body weight. Depending on the extent of the low energy balance coming out of summer heat stress, the effect on reproductive (and productive) performance will be more or less severe.

Heat stress also directly influences reproductive performance via altered follicle development, disturbed blood steroid concentrations, less estrous expression, endometrial dysfunction, lower pregnancy rates, and greater early embryonic mortality (Noordhuizen and Bonnefoy, 2015). Body temperatures in excess of 102.2oF appear to have detrimental effects on developing embryos from day one to six. During this period, the embryo is highly sensitive to heat stress.

The negative effects of heat stress carry over through fall, and even relatively short periods of heat stress compromise fertility. A primary reason for the carryover effect of heat stress on reproduction is explained by the fact that antral follicles need time to develop and once they are affected by heat stress, subsequent reproductive processes are delayed.

Heat Stress and Carryover Health Impacts

Heat stress affects health status of the cow – both in real time and after the heat has abated in the early fall. The impact on health is modulated through negative energy balance and compromised immune responsiveness (Noordhuizen and Bonnefoy, 2015). As a result, herds may experience elevated incidences of retained placenta, metritis, and mastitis even after the worst of the heat stress is finished.

Avoid Heat Stress During the Dry Period Too

In the past decade, researchers have realized that effective dry cow cooling results in far greater milk production once they calve (Tao and Dahl, 2013). On average, cows produce about 10 lb./d more milk if they are cooled during the dry period. In addition to greater milk yield, dry cow cooling improves immune status of transition cows and their calves, and it may improve even growth rate of the calf. Clearly, dry cow cooling during hot summer months is an overlooked opportunity to improve cow comfort and subsequent productivity when the cow calves in the fall.

Summary: Helping Cows Recoup from Heat Stress

To most effectively help cows recoup losses in production, reproduction, and health status – and to help mitigate negative carryover effects of heat stress that last into the fall months – focus on these nutrition and management priorities:

Feed highly digestible forage and non-forage sources of fiber. The focus must be on fiber digestibility. Rations should be formulated to optimize the supply of rumen fermentable carbohydrates – Table 1 supplies recommended ranges for total diet content of carbohydrate fractions that ought to optimize rumen digestive efficiency.

Manage for cow comfort to promote natural resting, standing, and feeding behavior. Above all, manage cows to avoid excessive standing time. Greater standing time during summer heat stress is correlated strongly with lameness in the fall – and lame cows will take far longer to recoup from the heat stress slump than sound cows.

Looking forward to 2017, plan now to effectively cool cows – both dry and lactating – to lessen the negative carryover effects of summer heat stress.

Dr. Grant points out that one of the top priorities in transitioning from summer to fall is to feed highly digestible forages and non-forage sources of fiber. If the forages you have on hand are not as digestible as you might like you have a couple of options: 1) purchase non-forage by-products or 2) add Amaferm to the diet. Amaferm maximizes the digestibility of the forages you already have on the dairy. Amaferm is economically beneficial by assisting you to feed more of your own forages as opposed to purchasing non-forage fiber sources.

Increasing the digestion of your own forages will:

Help cows gain weight lost during the summer

Allow you to feed a higher level of forage in the diet

Helping to maintain the 21-23% peNDF levels and the total NDF of 28-32%.